Method and apparatus for mobile data communication

ABSTRACT

A high-speed wireless transmission system is employable in a macro-cellular environment. In the system multiple transmit antennas are employed. Multiple carrier tones are used to transmit the data. The carrier tones can be assigned to the respective transmit antennas in such a manner as to provide each antenna with a subset of carrier tones with each subset being spread over the transmission spectrum. In addition, operation is enhanced by providing Reed-Solomon coding of the data across consecutive time intervals.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 08/718,718, filed on Sep. 24, 1996, now U.S. Pat. No.6,005,876, and entitled METHOD AND APPARATUS FOR MOBILE DATACOMMUNICATION.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for facilitatingmobile data communication, such as high speed data. The invention isspecifically related to a new arrangement for assigning carrier tones toa plurality of antennas and a coding technique to provide reliable,high-speed wireless access to mobile users in macrocells.

As more and more people come to rely on wireless communication and asInternet usage becomes more popular as well, it becomes desirable toprovide the ability for mobile wireless users to have multimedia accesssuch as to the Internet. However, effective multi-media access requiresa high-speed communication capability such as, for example, a bit rateof 1 to 2 Mbps.

It is currently known to provide a wireless data system with high bitrates over a short distance such as in a wireless LAN environment. Aco-pending provisional U.S. patent application, entitled CLUSTERED OFDMWITH TRANSMITTER DIVERSITY AND CODING, describes a technique forproviding such a high bit rate wireless LAN. In that technique an inputdata stream is encoded to allow for error/erasure correction in areceiver. Then, a multicarrier (or multitone) signal is formed. Formulticarrier, the basic idea is to divide the transmitted bandwidth intomany narrow subchannels that are transmitted in parallel. Eachsubchannel is then modulated at a very low rate to avoid significantintersymbol interference (ISI). The disclosed method employs OrthogonalFrequency Division Multiplexing (OFDM) a multiplexing techniquedescribed in for example, “Data Transmission by Frequency-DivisionMultiplexing Using the Discrete Fourier Transform” by Weinstein et al.,IEEE Trans. Commun. Technol. Vol. COM-19, No. 5, October 1971, pp.628-634 and “Multicarrier Modulation for Data Transmission: An IdeaWhose Time Has Come,” by Bingham, IEEE Commun. Mag., Vol. 28, No. 5, May1990, pp. 5-14. In the method disclosed in the provisional applicationgroups of adjacent tones are clustered together and separate clustersare provided to different ones of a plurality of separate independentantennas. A single receive antenna is then used to demodulate the OFDMsignal with conventional techniques.

A mobile data system has particular problems which limit the ability toprovide high speed multi-media access. The main impairments encounteredin a mobile radio environment are delay spread, doppler and path loss asrepresented by reduced received signal power. Delay spread refers to thefact that because the signal will experience a wireless path that willhave different impacts on different frequencies it is likely that theentire signal will not be received at the receiver at the same instantin time. A delay will be introduced. The delay spread in themacrocellular environment could be as large as 40 μsec which could limitthe data rate to about 50 Kbaud if no measures are taken to counteractthe resulting ISI. In the 2 GHz PCS bands the doppler rate could be ashigh as 200 Hz (i.e., a mobile unit moving at about 67 mph).Furthermore, the received signal power is inversely related to the datarate such that, for example, at a data rate of 1Mbaud (approximately 50times that of a typical voice circuit) there is a shortfall of at least15 dB in received power compared to cellular voice services and thiscreates a link budget problem. Thus, without any system modification thecoverage and performance of such systems will be severely limited. Infact, in the present wireless systems that cover a wide area with mobilereceivers, bit rates of 10 to 20 Kbps have been achieved. Therefore, itis desirable to adapt the wireless transmission systems to facilitatehigh-speed data communications.

SUMMARY OF THE INVENTION

The present invention achieves the desired high-speed wirelesstransmission by modifying the system to correct for the effects of delayspread and path loss. The present invention proposes an asymmetricservice: a high-speed down link (for example 1 to 2 Mbps peak datarates, or more) and a lower bit rate lower uplink (for example50-100Kbs). This would alleviate the problem of increasing powerconsumption at the mobile terminal to overcome the 15 dB shortfall inreceived power. Nonetheless, it should still be sufficient for mostapplications, such as Web browsing, voice access, e-mail, andinteractive computing.

Furthermore, the present invention provides an Orthogonal FrequencyDivision Multiplexing (OFDM) system that has narrow enough subchannelsand sufficient guard period to minimize the effects of delay spreads aslarge as 40 μsec.

To overcome the 15 dB shortfall in link-budget, the present inventionprovides transmit antenna diversity and coding across frequencies. Inone example the base station has four transmit antennas. Each antenna isassigned to transmit a subset of the total number of tones. A particularsubset is composed of a plurality of widely spaced tones covering theentire transmission bandwidth. As a consequence a subset of tones on asecond antenna will include tones between those transmitted on the firstantenna. Alternatively each subset of tones for a given transmit antennacan include widely spaced clusters of tones, e.g., two or three adjacenttones, which cover the entire transmission bandwidth. Spreading thetones over the transmit antennas randomizes the fading across the OFDMbandwidth.

The coding is also selected to help reduce the link-budget problem. Thedigital data are encoded using Reed-Solomon (R-S) encoding where symbolwords within R-S codewords are created by time-grouping modulationsymbols that are consecutive in time. The encoding uses a combination oferasure correction, based on signal strength, and error correction.

When the tone antenna assignment technique and the coding operation arecombined the link-budget problem is substantially alleviated.

In alternative embodiments the mobile station may include receiveantenna diversity. Also, the assignment of tones to the transmitantennas can be arranged such that the same tone is transmittedsimultaneously by two or more antennas. In yet another modification thetones assigned to a given antenna can be changed over time so that theeffect of any negative correlation between a given tone and a giventransmission path from a transmit antenna to a receive antenna can beminimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) respectively illustrate possible configurations of atransmitter and a receive station in a wireless LAN environment.

FIG. 2 illustrates a possible frequency characteristic of a giventransmission path from one transmission antenna to one receive antenna.

FIG. 3 illustrates in block diagram form an embodiment of the presentinvention.

FIG. 4 illustrates a graphical representation of a second aspect of thepresent invention.

FIG. 5 illustrates, in block diagram form, a more detailed embodiment ofthe present invention.

FIG. 6 illustrates how the embodiment of the present invention operatesto improve upon link budget shortfalls.

DETAILED DESCRIPTION

An example of a wireless transmission system in a LAN environment suchas described in the above-referenced provisional application isillustrated in FIGS. 1(A)-(B). A data bit stream is provided to anencoder 101 which produces a plurality of symbols. In this instance theencoder produces N×M symbols. N tones are assigned to each of Mantennas, e.g., 104 ₁ . . . 104 _(M). The first N tones are provided toIFFT (Inverse Fast Fourier Transformers) 102 ₁ while the Mth group of Ntones is provided to IFFT 102 _(M). Each group of tones is provided toRF circuitry (e.g., filter and amplifier), e.g., 103 ₁ . . . 103 _(M)and then passed on to its respective transmit antenna 104. The totalnumber of tones (N×M) equals the total number of carriers in themulticarrier OFDM configuration. The carriers are spread out over thetransmission spectrum.

Graphical representations are provided adjacent to each antenna 104 ₁ to104 _(M) to illustrate that a given antenna is assigned a particularcluster of adjacent tones or carrier frequencies. Each cluster is partof a very localized portion of the overall transmission spectrum.

The M clusters of tones are transmitted from the M transmit antennassimultaneously and received by the receive antenna 110. As can be seenfrom the adjacent graphical representation, the antenna receives all ofthe clusters nearly simultaneously.

The antenna provides the received multicarrier signal to RF circuitry111 which then provides the processed signal to FFT 112. The resultantdata corresponds to the N×M symbols produced by the encoder 101 in FIG.1(A) and the decoder 113 receives these symbols and provides as anoutput the data bit stream. What is not shown in the graphicalrepresentation is that the receive antenna 110 may receive differentfrequencies at different strengths. In the mobile environment multi-pathpropagation may pose a significant problem for such a configuration suchthat certain of the frequencies may be so seriously faded as to haveessentially dropped out.

It is considered that the channel from one of the transmit antennas tothe receive antenna can be different from the channel from anothertransmit antenna to the receive antenna. These are considered separateand distinct paths. Each path has its own frequency responsecharacteristic. For instance, as illustrated in the graphicalrepresentation of FIG. 2, a first path may more successfully transmitfrequencies in the range of f₂ while having more difficulty transmittingfrequencies in the range of f₀ and f₁. Thus, the inventors recognizedthat for a number of tones in the region of f₀ (or f₁) if those tonesare clustered together and are the sole cluster provided along antenna1, then the signal from antenna 1 will either be difficult to detect atthe receiver or will likely contain many errors due to the path'scharacteristics.

To remedy this problem the inventors propose to provide a spreading outof the carrier tones across the transmission spectrum. This willcounteract any frequency dependence that a particular path might have byoptimizing the chances that each path will substantially transmit usefuland correct information.

As illustrated in a first embodiment of the present invention shown inFIG. 3, a sequence of data bits is provided to a modulator 301 thatcreates a modulator symbol. The modulator symbols are grouped, encodedand subsequently decomposed by elements 302, 303 and 304 as will bedescribed in further detail later. The resultant data stream isconverted to a parallel stream of data by serial-to-parallel converter305. As an example, 120 symbols of data are provided in parallel (X₀ toX₁₁₉). Where the symbols are QPSK modulated each consists of two bits.Other modulators can be used to create either symbols such as 8-PSKsymbols. A distributor receives this 120 parallel symbol block of data.Each of the 120 symbols corresponds to one of 120 carrier tones to beused in the multi-carrier OFDM configuration. The distributor can sendclusters of symbols to each of M IFFTs (307 ₁ to 307 _(M)). In thepresent example the distributor sends groups of five symbolscorresponding to five carrier tones to each of the IFFTs. In the presentexample it is proposed that M=4 so that there are four transmit antennas(309 ₁ to 309 _(M)) providing four separate paths to a single receiveantenna. The distributor therefore provides thirty symbols of the 120symbols to each of the transmit antenna paths. It does so in five-symbolclusters that are spread out over the entirety of the transmissionspectrum, i.e., IFFT 307 ₁ receives symbols at X₀ to X₄, X₂₀ to X₂₄, X₄₀to X₄₄ . . . X₁₀₀ to X₁₀₄. Similarly, the remaining IFFTs also receiveclusters of symbols assigned to carrier tones that are spread over thetransmission spectrum. Therefore, as shown in the graphicalrepresentations associated with antennas 309 ₁ and 309 _(M) in FIG. 3,the first antenna sends out six clusters of tones. These tones arespread over the entire transmission spectrum. As can be seen the sixclusters transmitted by antenna 309 _(M) are interleaved with theclusters transmitted by 309 ₁. Although not shown, the clusters for 309₂ and 309 ₃ (where M=4) are also interleaved with the clusters of tonesto be transmitted on the other antennas.

In summary then, the problem of the frequency dependence of a given pathfrom one antenna to the receive antenna and the path's susceptibility toadversely affecting the overall transmission characteristic when it onlytransmits a cluster of the multiple carrier tones, are overcome byproviding a subset of the carrier tones to each of the antennas wherethe subset for a given antenna is spread over the entire transmissionspectrum. As a consequence, not all of the tones on a given antenna areadjacent to one another. In fact, where tones on antenna 1 are notadjacent to one another (e.g., tone for X₄ and tone for X₂₀), there areintermediate tones which are supplied by different ones of the transmitantennas.

Modifications to this arrangement may be desirable. For instance, in theexample described above the distributor 306 receives 120 symbols anddivides them among four transmitters. Each cluster within a subset oftones can be constituted by a single tone rather than a group ofadjacent tones. Thus, one possible modification to the arrangement ofFIG. 3 would assign the tones corresponding to symbols X₀, X₄, X₈, X₁₂,X₁₆ etc. to antenna 1 and the tones for symbols X₁, X₅, X₉, X₁₃, etc. toantenna 2 and so on. This arrangement should achieve a substantiallysimilar result in that the improvement arises from the spreading out ofthe tones for a given antenna over the entire transmission spectrum andinterleaving the tones carried by various ones of the antennas.

In another modification to improve received signal strength at thereceiver it might be appropriate to send the same signals on multipletransmitters. In this instance, it is conceivable to employ, forexample, eight transmit antennas where each antenna is separate anddistinct and thus provides different paths each having their owncharacteristics. Then, the same configuration as described with respectto FIG. 3 could be employed with the change being that the same outputstream provided to 309 ₁ could also be provided to another antenna sothat two transmit antennas would be responsible for transmitting symbolsX₀ to X₄, X₂₀ to X₂₄ and so on. This could improve the overall receivecharacteristics.

In yet another modification to this design it is possible to vary thetone assignment among the transmit antennas. As an example, should thepath associated with transmit antenna 309 ₁ have characteristics whichare adverse to the tones for symbols X₀ to X₄ this problem can bealleviated by rotating the assignment of the clusters of tones among thevarious antennas. Therefore, in a first instance a first block of 120symbols might be assigned in the manner illustrated in FIG. 3. A secondblock could be transmitted with a different set of tone assignments,e.g., 309 ₁ receiving tones for symbols X₁₅ to X₁₉, X₃₅ to X₃₉, etc.This changing of the assignment of tones to a given transmit antennaassists in avoiding potential adverse impacts of a given antenna'stransmission characteristic upon any of the carrier tone or toneclusters. This could be accomplished by inserting a switchingarrangement between the IFFT's (307 ₁-307 _(M)) and the RF circuitry sothat the IFFT's are alternately assigned to the respective antennas.

Of course, one of ordinary skill in the art would recognize that giventhis description of various modifications to the first embodiment ofFIG. 3 that combinations of these modifications would also be possible.For example, the rotation of carrier tones among transmit antennas couldalso be performed when transmitting individual tones rather thanclusters of tones.

The inventors also recognize that the way the coding is done can have apositive influence on the word error rate to thereby further address thelink budget problem. In particular, the inventors have selectedReed-Solomon (R-S) encoding. In an example of such an encoding schemeeach R-S symbol is constituted by six bits of information and the R-Sblock is constituted by a predetermined number of R-S symbols with acertain subset of those symbols being directed to data symbols and theremaining being directed to parity symbols. As is known, whether thereis one bit of an R-S symbol which is in error or whether there aremultiple bits of the R-S symbols that are in error, it takes two paritysymbols to correct each R-S symbol in error unless the location of theR-S symbol that is in error is known. In the latter circumstance such anR-S symbol is considered an erasure and only one parity symbol isnecessary to correct such an error. To enhance the through-put of datait is desirable to keep the number of parity symbols low. However, toaccomplish this goal it is beneficial to construct the R-S data symbolsin a manner that maximizes the concentration of errors, i.e., ratherthan spreading out data bit errors over multiple symbols it is desirableto increase the likelihood that those bits that will be in error will bein the same R-S symbol.

The inventors have determined that the optimum way to concentrate thesebit errors is to group the modulator symbols in time rather than byfrequency. As shown in the example of FIG. 4, there are three blocks ofmulticarrier signals shown at different times each with a time width of200 microseconds. The frequencies f₁ to f_(S) correspond to themulticarriers over the transmission spectrum. The inventors discoveredthat it is beneficial to construct a given R-S symbol, for exampleR-S_(x) from three symbols of the same frequency over the threeconsecutive time periods. Thus, for example, R-S_(x) would be comprisedof frequency f_(x) at time t₁, f_(x) at time t₂ and f_(x) at time t₃.The actual construction of these Reed-Solomon symbols and code words aredescribed in relation to FIGS. 3 and 5.

As illustrated in FIG. 3, the modulator symbols at the output of themodulator are provided to a symbol grouper 302. An example of a sequenceof data bits is shown at 51 in FIG. 5. In one more specific embodimentof the present invention the modulator 301 is a serial QPSK (QuadraturePhase Shift Key) modulator. In this embodiment the modulator converts ablock of 360 data bits into 180 2-bit symbols (d₀ to d₁₇₉). Each R-Ssymbol is six bits in length, thus three QPSK symbols can be grouped toform a single R-S symbol. In accordance with the inventors' discoveryregarding time grouping of symbols as illustrated in FIG. 4, threesymbols consecutive in time rather than frequency can be groupedtogether to form the R-S symbol. For instance, where there are 180 2-bitsymbols there are three blocks of sixty 2-bit symbols: d₀ to d₅₉ attransmission time t₁, d₆₀ to d₁₁₉ at transmission time t₂, and d₁₂₀ tod₁₇₉ at transmission time t₃. Thus, a grouping in time of three 2-bitsymbols to create an R-S symbol could be effected by grouping symbolsd₀, d₆₀ and d₁₂₀. The R-S coding shown in FIG. 3 would result in threesets of forty R-S symbols with each set containing 20 data symbols and20 parity symbols. The decomposer 304 would then reconfigure the QPSKsymbols within the R-S words in time to create time blocks oftransmission symbols, e.g., z₀ to z₅₉, z₆₀ to z₁₁₉, and z₁₂₀ to z₁₇₉. Aserial to parallel converter 305 takes the symbol stream and creates aparallel configuration of 120 symbols at a time. A distributor 306 thendivides the 120 symbols among the multiple transmit antennas inaccordance with the carrier tone assignment for that given antenna inaccordance with the discussions above.

Thus, in this exemplative arrangement there are 120 tones with a 160μsec block size and a 40 μsec guard. This results in subchannels thatare spaced by 6.25 kHz, block rates of 5 kbaud, and a total rate of 600kbaud or equivalently channel bit rates of 1.2 Mbps for QPSK.

The combination of the coding technique with the assignment of tones tothe various transmit antennas has shown an ability to substantiallyovercome the link budget problem described above. As shown in FIG. 6where R-S encoder provides 40 symbol words each word including 20 paritysymbols with 20 time grouped data symbols, a desired word error rate WERof 1% requires less than 8.5 dB signal to noise ratio rather than the 17to 20 dB which is typically required for cellular systems. Thisrepresents about a 9 dB reduction in the link budget shortfall discussedabove. This significantly improves the ability to transmit high speeddata in the wireless environment.

In connection with the actual error detection at the receiver end, andwith a goal in mind of maximizing the use of the parity symbols it ispossible to designate a percentage of the parity symbols as beingrelated to correcting erasures and the remaining being directed tocorrecting errors. For example, where there are 20 parity symbols it ispossible to correct ten erasures (one symbol per erasure) and fiveerrors (two symbols per error). To accomplish this end in a given R-Sword the algorithm can designate that the ten least powerful R-S symbolscan be treated as erasures and corrected as such. Then five additionalerrors could be corrected if they exist in any of the remaining R-Ssymbols. Other criteria for estimating that an erasure has occurred canbe employed such as measuring the bit error rate or using an “innercode” (an error detection code) to detect where errors occur.

The exemplative embodiments illustrated in FIGS. 3 and 5 can beconstructed using well known components. First, the R-S encoder can beconstituted by the Reed-Solomon Error Correction Device sold by AdvancedHardware Architectures of Washington State.

The signal processing functions can be implemented in a DSP whichprovides IFFTs as is well known. The time grouping and decomposing canbe implemented using buffers. For example, to group-in-time a buffercould store 120 2 bit symbols and then those symbols could be read outin a predetermined time order. Similarly for the decomposer the R-S codewords could be stored in a buffer and the individual 2-bit symbolswithin the code word could be read out in a predetermined order. Also,the distributor can take the form of a de-multiplexer in that it takesthe symbols from the Serial/Parallel converter and passes selectedsymbols to selected IFFTs which is simply the dividing up of informationbetween channels, a common de-multiplexing function.

To further enhance the receiving characteristics at the mobile station,it is possible to employ multiple antennas, e.g., two. The signals fromthe two antennas can then be combined so as to further reduce thelikelihood that any significant number of the carrier tones isinsufficiently received.

In the foregoing the Applicants have described two techniques which canbe employed in connection with the wireless transmission of data toincrease bit rate, namely the assignment of carrier tones to multipletransmit antennas with the carrier tones assigned to any one antennabeing spread over the transmission spectrum and a particular type ofcoding technique. These two aspects can be employed separately or theycan be combined together to further improve the achievable bit rate.

What is claimed is:
 1. A method for high-speed wireless transmission ofdata over a transmission spectrum comprising the steps of: creating astream of data; encoding said stream of data to create a plurality ofsymbols; assigning each of said plurality symbols to one of a pluralityof carrier tones; providing each of said carrier tones to one of aplurality of transmission antennas, in such a way that each antennareceives a subset of said plurality of carrier tones and each subset ofsaid plurality of carrier tones includes at least two carrier tones notadjacent to one another in the transmission spectrum and having at leastone carrier tone therebetween that is provided to another one of saidplurality of transmission antennas; and simultaneously transmitting thesubsets of carrier tones from said plurality of transmission antennas.2. The method of claim 1 wherein said step of encoding comprises thesubsteps of: grouping said data in said stream to create multi-bitcoding symbols, each coding symbol containing a plurality of modulationsymbols grouped in time; and generating a code word from a plurality ofcoding symbols.
 3. The method of claim 1 wherein in one subset of saidplurality of carrier tones the carrier tones are uniformly distributedover the transmission spectrum.
 4. The method of claim 3 wherein saidstep of encoding comprises the substeps of: grouping said data in saidstream to create multi-bit coding symbols, each coding symbol containinga plurality of modulation symbols grouped in time; and generating a codeword from a plurality of coding symbols.
 5. The method of claim 1wherein in one subset of said plurality of carrier tones the carriertones are distributed over the transmission spectrum in a non-uniformmanner.
 6. The method of claim 5 wherein said step of encoding comprisesthe substeps of: grouping said data in said stream to create multi-bitcoding symbols, each coding symbol containing a plurality of modulationsymbols grouped in time; and generating a code word from a plurality ofcoding symbols.
 7. The method of claim 1 wherein at least two subsets ofsaid plurality of carrier tones on separate antennas include the samecarrier tones.
 8. The method of claim 1 wherein for a given one ofplurality of transmission antennas a first subset of carrier tones at afirst time and a second subset of carrier tones at a second time includedifferent carrier tones.